Mobile computing devices have been widely adopted in recent years. Many functions previously performed primarily by personal computers, such as web browsing, streaming, and uploading/downloading of media are now commonly performed on mobile devices. Consumers continue to demand smaller, lighter devices with increased computing power and faster data rates to accomplish these tasks. Additionally, mobile devices increasingly need to support the large number of frequencies specified by the various communications standards, and therefore, larger number of antennas need to be supported.
The allocated space for one or more antennas is called antenna volume or antenna keepout. However, due to established theoretical limit on antenna performance based on antenna keepout, design of multiple antennas in a device would add to the overall size of the device, which may not be desirable. Another constraint in the device and antenna keepout design is the interaction (or coupling) between the different antennas. For example, coupling between two antennas causes problems such as interference, efficiency/gain degradation, and detuning, which would further complicate multi-antenna system design and configuration.
Multi-antenna configurations, including antenna diversity (diversity) configurations and multiple-input, multiple-output (MIMO) configurations, have been used in attempts to increase the quality and data rates within a constrained spectrum of wireless communications. Antenna diversity refers to configurations that transmit or receive multiple versions of a signal to increase the likelihood that the signal will be received without errors or noise. The principle behind diversity configurations is that circumstances that adversely affect one version of a signal may not affect another version of the signal. Diversity includes, for example, time diversity, in which a signal is transmitted/received at different times; frequency diversity, in which a signal is transmitted/received at different frequencies; spatial diversity, in which a signal is transmitted/received from/at different positions; and polarization diversity, in which a signal is transmitted/received at different polarizations. Diversity configurations of two receive antennas and one transmit antenna, for example, are possible. Other configurations including multiple transmitters and/or receivers are also possible and may be used in some embodiments.
Diversity alone, however, does not necessarily affect data rates. Rather than using multiple antennas only to provide an additional signal source to improve accuracy of a signal, MIMO systems increase data rates by using multiple antennas that act together to transmit more information. MIMO can include: multi-stream beam forming in which signals received at different antennas add constructively; spatial multiplexing in which each of a plurality of transmit antennas transmits a signal at the same frequency but using a lower data rate, and the transmit signals are combined on the receive end; and using multiple antennas to transmit orthogonally coded versions of a single bitstream at each of a plurality of antennas. MIMO can be viewed as a type of diversity. Even with the adoption of diversity and MIMO configurations, further advances are needed in antenna design and configuration.